The Methanococcus aeolicus enzyme was purified to homogeneity and characterized [Xing94]. It was initially reported to be a homodimer composed of a single type (large) subunit, although subsequent work has showed that the gene encoding the large subunit is followed by a gene encoding a small subunit, suggesting that the enzyme is actually composed of two types of subunits, similarly to the Escherichia coli enzyme [Bowen97].